It is known in the art that all carcasses of pneumatic green tires are built as a series of layers of flexible high modulus cords encased in a low modulus rubber; the cords in each layer are oriented in a chosen path or direction and substantially equispaced and parallel. Before curing, the tire is often shaped by blowing air inside, and at that time substantial expansion occurs. Components with lower low strain modulus expand easier than components with higher low strain modulus. The tire, whether belted radial ply or bias ply, is cured in a curing press using a curing bladder which forces expansion of the tire. When the carcass is cured with an innerliner in it, as it usually is, the innerliner may further expand with the carcass which is forced against the indentations in the curing mold to form the tread, and all components are co-cured so as to provide a substantially cohesive bond between one and another.
An innerliner for a pneumatic tire is typically formed from either a compound containing a major proportion by weight of a halobutyl rubber, or a compound consisting primarily of all natural rubber. Before the tire is cured, the entire original inner surface of the innerliner and/or the outer surface of a shaping bladder used in the curing press is coated with a release agent. The release agent is commonly referred to as a “lining cement” when used on the surface of the innerliner; and, to a “bladder lube” or “bladder spray” when used on the shaping bladder.
The surface of the innerliner, or the interior surface of an innerliner-free green carcass, or the exterior surface of the tread, or the exterior surface of the sidewall, protected so as never to have come in contact with release agent, is referred to as a “virgin surface” whether it is cured or not. Such a virgin surface permits a rubbery article to be bonded to it without having to clean the surface; in most instances, such bonding to a contaminated surface is typically done after cleaning it, first by scrubbing/buffing with a wire brush in combination with an appropriate solvent, followed by vacuuming the solvent. Such cleaning is necessary to remove the lining cement or bladder spray (release agent), typically an organopolysiloxane or “silicone” based material, such as poly(dimethylsiloxane) including powdered mica or crystalline silica and afford a “cleaned surface”. Cleaning is both time-consuming and environmentally unfriendly, since the solvent is non-aqueous and aggressive; moreover, its use is restricted. Nevertheless, before automobile tires are provided with a puncture sealant and/or balance pads, noise-reducing foams are applied to the inner surface of a cured innerliner, it must be thoroughly cleaned. Cleaning to get rid of contaminating release agent so as to provide a “cleaned surface”, has been done over several decades, and is still done.
Alternatively, the release agent may be removed by washing with an appropriate detergent, or mechanically, by buffing or abrading the surface until the contaminant is removed. Since tire manufacturers were inured to the disadvantages and additional cost of a cleaning step, they were unaware, until very recently, that bonding a rubbery tire component to a cured virgin surface of a tire provides an unexpectedly stronger bond than bonding to a “cleaned surface”, even if it is meticulously buffed and solvent-cleaned.
Providing an uncontaminated tire surface is important because a carcass of a pneumatic rubber tire, whether a radial or bias ply, is often required to have a rubbery component bonded to a portion of the tire's surface, either exteriorly on the sidewall, or internally within the toroid. For example, an aircraft tire, typically of bias ply construction, is dynamically balanced by adhering a laminar pad of rubbery material, referred to as a “balance pad”, symmetrically about the circumferential centerline of the interior surface of the cured tire. However, because the precise position at which the balance pad is to be adhesively secured cannot be determined until the tire is cured and dynamically balanced, the circumferential crown area of the entire innerliner is kept clean by the film method as described in U.S. Pat. No. 7,332,047. By definition, the crown area of the innerliner is the portion of the innerliner located underneath the tread.
A balance pad, such as one commercially available from Patch Rubber Company, is a multilayer rubbery component which typically includes (i) a thick layer of high specific gravity compound blended with iron oxide and cured, the thickness and/or area being a function of the weight desired, (ii) a relatively thin layer of high elongation floater gum or stretch ply of rubber filled with carbon black, (iii) a bonding gum layer (also referred to as a “gray-face gum” layer) of curable rubber compound with curing agent but without a cure accelerator or activator fluid, and (iv) a protective film covering the bonding gum layer. When the protective layer is removed from a balance pad of desired weight, and the exposed bonding gum layer is secured with a fast-dry cement containing a cure-accelerator, typically an alkylamine or arylamine, to the rubbery surface of a cured innerliner, or of an innerliner-free cured carcass, the curing of the bonding gum layer to the rubbery surface, typically at ambient temperature over a period of several days, ensures that the balance pad will not be dislodged during operation of the tire. However, since one cannot know in advance where the balance pad will need to be positioned, circumferentially a certain width of the crown area of the innerliner is kept clean.
To protect the entire virgin surface of either a portion of the exterior surface of the tire, or an innerliner, logic dictated that the virgin surface be protected from a conventional release agent in the first place, and that this be done by a removable barrier film between the curing bladder and the virgin surface while the tire was being cured, the barrier film to be readily removable after curing.
The problem in implementing the logical choice, that is, to counter contamination of the entire innerliner surface by the already-present release agent at a temperature in the range at which the tire is to be cured; in some instances only a minor portion of the innerliner may need to be protected. Choice of a barrier film dictated that it be heat-resistant in that temperature range, typically from about 121° C. (250° F.) to 200° C. (392° F.).
Further, expansion of a green carcass in the curing press dictates that, to protect a major portion of the innerliner's surface, the barrier film be extensible at least 2% at curing temperature in any direction on the surface the barrier film is to protect, and to stretch during curing without tearing. A green belted radial ply tire for an automobile expands in the curing press in a range from about 1% to 20%; a conventional green cross bias casing of a bias ply tire with a crown angle in the range from 20° to 38° expands in the curing press in a range from about 20% to 250%, expansion of aircraft tires being greatest. Therefore, a usable barrier film is required to be adequately expandable within the curing carcass, that is, multiaxially or uniaxially expandable, without tearing in the range from about 5% to 100%. The barrier film is also required to be adequately thermoformable, in that it conforms to the shape of the bladder during curing, thus squeezing out entrapped air, and after being thermoformed the film substantially retains its formed shape as the film has essentially no memory and is non-elastomeric. In the instance when the entire virgin surface is to be protected, one end of the barrier is overlapped over the other (which other end is applied or “stitched” to the virgin surface to be protected and secured with a pressure-sensitive adhesive layer at the interface of the overlap) so as to form a “pull-tab” for easy removal.
Still further, since a substantial period of time may elapse before the cured tire is taken up in a production line to have a desired component adhesively secured to it, it is desirable that, before the tire is cured, the barrier film be secured to the virgin surface of the cured tire, whether it has an innerliner or not, sufficiently well that only a small force in the range from about 0.4 to 7.9 N/cm (1 to 20 N/inch) is required to remove the barrier film. Moreover, it is essential that, after the tire is cured, the barrier film remain on the innerliner and not fall off into the tire mold.
Substantially the same problem was addressed in U.S. Pat. No. 4,443,279 to Sandstrom, which was directed to “a pneumatic rubber tire, and method of preparation, characterized by having a co-vulcanized, removable, rubber innerliner adhering to the inner surface of the tire, said innerliner comprised of a sulphur cured carbon black filled rubber compound of (A) butyl rubber and (B) an ethylene/propylene/nonconjugated diene terpolymer. The information further relates to such pneumatic rubber tire in which its exposed inner surface is provided by removal of said innerliner. The information has a particular utility in providing a pneumatic rubber tire with a clean, exposed, inner surface.” However, tests indicate that when the illustrative example was duplicated the strip was not readily removable (it tore when it was being removed), and when one end of the cured strip overlaps the other, the ends become fused together upon curing and cannot form a pull-tab for manual removal. The major disadvantages of using such a compound is that it needs several Banbury mixing steps, then a calendaring step, and a wrapping film for calendering. Moreover, calendering is difficult, if not impossible, if thickness is less than 30 mil. Thus, when the removable layer is removed, it generates a lot of cured rubber for disposal or waste. Another disadvantage is that it stretches during removal, breaks, and comes out in several pieces.
One skilled in the art will know that films of numerous synthetic resinous compounds such as Mylar® polyester, Saran® (poly(vinyl chloride)-co-vinylidene chloride), cellophane, polyurethane and polyolefins such as polyethylene (PE) and polypropylene (PP), can be “stitched” with varying degrees of success, onto the exterior of, or into the interior of a green tire because the uncured rubber is tacky enough to do so. Even a heavily cured (high cross-link density) strip of rubber may be stitched into, and remains positioned in the interior, though not reliably; and upon curing, the strip is readily removable, but it too-often tears in the mold because it does not expand sufficiently, and is usually removed in pieces; having been rent, it fails to protect the virgin surface from contamination by the bladder lube coated on the curing bladder. Further, if the curing bladder is not coated, it will adhere to the portions of the carcass where tears in the cured strip have occurred, damaging the bladder when the carcass is torn from it.
Even substituting a cured thin first strip for the uncured strip used by Sandstrom, fails to provide an effective barrier layer because the pre-cured strip tears upon removal. Substituting a less heavily cured (lower cross-link density) second strip which will not tear (and is more readily stitched into the interior of the green carcass than the cured), provides the necessary expansion and excellent protection when the tire is cured—but the second strip still adheres to the protected surface too tightly to be removed integrally, and cannot be easily removed.
As shown in U.S. Pat. No. 7,332,047 to Majumdar et al., a barrier film chosen from readily available films of precured and cured rubber, Mylar, Saran, polyurethane, cellophane, PE and PP was ineffective. Though thermally stable at curing temperature, Mylar and cellophane films wrinkle in the mold because they do not expand, and lining cement enters underneath. They are effective only when a portion of the innerliner is to be protected, provided the film stays in position. In practice, the film becomes dislodged and falls off in more than 10% of cured samples which is unacceptable. Saran®, PE, and PP are melts at tire-curing temperature. Hydrogen chloride generated by decomposition of Saran contaminates the mold. A polyurethane strip less than 5 mils thick is too rubbery to pull out off the tire.
An improved barrier film was presented in U.S. Pat. No. 7,332,047, wherein a removable “self-supporting barrier film of non-sulfur vulcanizable, expandable, thermoformable synthetic resinous material” is applied to the surface of the tire innerliner prior to curing to prevent release agents from contaminating the tire's “virgin surface.” Unlike compounded rubber barrier of U.S. Pat. No. 4,443,279, one of the major advantages of U.S. Pat. No. 7,332,047 is that the barrier film can be made very thin (e.g. 1 mil) by a single extrusion step, thus significantly reducing the solid waste generated when it is removed and discarded. Due to its high low strain modulus (50% modulus is 35.75 MPa), it does not stretch much during removal and comes out easily in one piece as the tensile strength is high. U.S. Pat. No. 7,332,047 noted that the “adhesion of a cured rubbery component to a virgin surface of rubber never contaminated by the remnants of a release agent (such remnants are left by solvent-cleaning or buffing the release agent off a cured innerliner surface to leave a cleaned surface), is several-fold stronger than a bond of the cured component to the contaminated, then ‘cleaned surface.’”